Adobe Flash Player is an obsolete technology. Adobe has announced that it will stop Flash Player by the end of 2020. Firefox is trying to get rid of this inheritance and reduce its support.

From version 52-53 Firefox has restricted Flash and no longer gives Abobe Flash video links to the AntDM extension.

In some cases Adobe Flash Player uses the HTTP protocol, so you can enable AntDM's WPCap support (Config\General Tab) to capture video links but this does not work for HTTPS links (or not on secure addresses).

There is no problem with Chrome or other Chrome-based browsers: Opera, Vivaldi, Yandex, ...